Antireflection coating films for optical elements are mainly formed on glass and plastic. Optical elements may be lenses, prisms, or another optical glass. Here, an optical element will be described by taking a lens as an example.
As illustrated in FIG. 1, an antireflection coating film 1 for an optical element may be formed on any portion of the circumference of a lens 2 serving as an optical element. Like incident light 3, in the case where light is incident on only the lens 2, the light comes through the lens 2 to emerge as transmitted light 4. In contrast, in the case where obliquely incident light 5 enters the lens 2, the light is incident on the antireflection coating film 1. In this case, if the antireflection coating film 1 is not arranged, light incident on the circumference of the lens 2 undergoes inner-surface reflection to emerge as internally reflected light 6 that is not related to an image from the lens 2, thereby causing, for example, flare and a ghost image and degrading the image. The arrangement of the antireflection coating film 1 reduces the inner-surface reflection of the obliquely incident light 5 to reduce the internally reflected light 6 that adversely affects an image, thereby preventing the formation of flare and a ghost images.
In recent years, with a trend toward miniaturization and higher performance of lenses, lenses with high refractive indices have been developed. Higher refractive indices of lenses require higher refractive indices of antireflection coating films.
With respect to a method for preventing inner-surface reflection, PTL 1 discloses a method in which coal tar is used to improve a refractive index and in which light is absorbed by the color of coal tar itself. Furthermore, PTL 2 discloses a method in which an antireflection coating film having a higher refractive index is provided using inorganic fine particles having a high refractive index and in which light is absorbed by black fine particles.